For a common induction-melting apparatus, use is made of a relatively small-sized apparatus of a type using a crucible formed of refractories made of sintered magnesia or alumina, provided inside an induction coil, or a large-sized apparatus comprising a crucible layer formed by sintering the surface layer of powdery refractories. With any type of those apparatuses, it is a common practice to use the same after filling refractory powders in-between the induction coil, and the crucible layer as sintered in order to protect the crucible from high temperature.
For induction-melting of material having a high melting point, such as iron-base alloy, nickel-base alloy, or cobalt-base alloy, a crucible made of refractories excellent in refractoriness at high temperature is required. Further in the case of using a melting apparatus of a water-cooled copper coil type, there is no denying a possibility that a crucible undergoes melt-damage, thereby directly affecting the copper coil.
Further, upon melting for refining an alloying material of high-purity, and high cleanliness, if a large amount of a halide-base refining flux is used for the purpose of desulfurization, and dephosphorization in a molten metal, there arises a risk that the flux in a molten state at a high temperature will cause the melt-damage of refractories making up a crucible, such as silica, alumna, magnesia, or calcium oxide, and so forth.
Accordingly, if a cold-crucible induction-melting method using a crucible made up of water-cooled type copper segments is applied, this will cause temperature of the crucible during a melting operation to become as low as around 200° C., so that it is possible to prevent consumption of the refractories due to the melt-damage thereof, caused by the refining flux in the molten state at the high temperature, as described above. Therefore, even in the case of using the halide-base refining flux, a permissible range of composition thereof will become greater.
In each of the following Patent Documents 1 to 3, there is disclosed an excellent method for refining stainless steel through desulfurization, and dephosphorization, using such a system as described in the foregoing, however, even with this method, another problem caused by the water-cooled copper crucible is encountered. More specifically, if an operation is carried out in the copper crucible cooled down to on the order of 200° C., in a state of a molten metal and a refining flux being in direct contact therewith, a temperature of the refining flux becomes excessively low, thereby impairing refining effects. Or an amount of heat transfer from a molten pool to the copper crucible becomes excessively large, leading to deterioration in power efficiency. In order to cope with such problems as described, techniques devised so as to enable adequate control of an operation for water-cooling the copper crucible, and so forth are conceivable. However, a finely controlled operation cannot necessarily be executed with ease owing to latent problems such as constraints from the viewpoint of a water-cooling structure of the copper crucible, constraints from the viewpoint of safety of the copper crucible, and so forth.
In Patent Document 4 described hereunder, there is disclosed a crucible structure devised in such a way as to enable induction-melting to be effected in a cooled state higher in temperature than the water-cooled copper crucible. More specifically, there is adopted a structure wherein a plurality of cooling pipes are disposed inside the wall of a heat-resistant crucible made of alumina, or magnesia, the outer periphery of the crucible is covered with an electrically conductive airtight sealing cylinder, and an induction coil is disposed on the outer periphery thereof. Further, in another Patent Document 5, it is described that refractory ceramic coating of yttrium oxide, and so forth is applied to the inner wall of a crucible of an induction-melting apparatus, thereby executing the so-called cold crucible levitation melting.
After all, with a method whereby the cold-crucible melting method is combined with flux refining, problems remain in that there occur not only deterioration in the refining effect itself, attributable to a drop in temperature of the refining flux due to a cooling action, but also deterioration in the power efficiency, due to a large amount of heat transfer from the molten metal. Further, with the heat-resistant crucible provided with the cooling pipes, disclosed in Patent Document 4, a halide-base flux will react with the refractories of the crucible, so that it is difficult to make use of refining using flux. A method for applying mold wash of refractory ceramics to the crucible, as disclosed in Patent Document 5, is not suitable for use when applying mold wash with a refining flux having high refining performance at a low melting point.
Further, the invention relates to a method for melting ultrahigh-purity alloying materials for Fe-base, Ni-base, and Co-base alloys, respectively, as represented by stainless steel, and various superalloys.
As requirement quality is more diversified, or becomes higher in grade, it is a major problem emerging upon melting of an ultrahigh-purity alloying material such as a Fe-base, an Ni-base, or a Co-base alloying material that impurity elements such as oxygen (O), nitrogen (N), sulfur (S), phosphorus (P), and so forth need be comprehensively reduced to the utmost limit.
A conventional mass production low carbon stainless steel is produced through a process of electric furnace—AOD (VOD)—ladle refining, and so forth, and a normal level of those C+O+N+S+P base impurity elements is on the order of 400 ppm, the reduction limit thereof being considered around 250 ppm.
In contrast, as a method for producing a ultrahigh-purity stainless steel, and an Ni-base superalloy, there is a well known method for executing melting in two stages, whereby a vacuum induction melting method is applied in primary melting, and a vacuum arc melting method, an eletro-slag melting method, or an electron-beam melting method, and so forth are applied in secondary melting. It is known that if a ultrahigh-purity and high Cr—Ni austenite stainless steel is produced by the vacuum induction melting method (the primary melting), and the electron-beam melting method (the secondary melting), the level of those C+O+N+S+P base impurity elements can be lowered to on the order of 100 to 150 ppm.
However, in the case where very high corrosion resistance is required as is the case with the latest equipment intended for a nuclear power plant, supply of an ultrahigh-purity alloying material is required, and there is a demand for a new technology capable of further reducing levels of impurities such as S, P, and so forth. In the case of producing this kind of alloying material containing various alloy constituents, many impurity constituents are generally brought in from alloy raw materials to be added. In particular, various raw materials as supply sources of Cr, Mn, and so forth, in heavy use as the alloy raw materials, contain much of impurity elements such as S, P, C, O, N, and so forth. A representative raw material for electrolytic chromium contains C: 130 ppm, O: 440 ppm, N: 45 ppm, P: 10 ppm, and S: about 26 pm and raw material for electrolytic manganese contains C: 40 ppm, O: 1600 ppm, N: 50 ppm, P: 10 ppm, and S: about 260 pm. Accordingly, even if use is made of a high-purity raw material for iron (electrolytic iron), and a high-purity raw material for Ni (electrolytic nickel), in order to melt those alloy raw materials, containing Cr, Mn, and so forth, in large amounts, it is necessary to effectively remove those impurity elements such as S, P, C, O, N, and so forth out of a molten metal as adjusted to match alloy composition by refining.
Now, the vacuum induction melting method often adopted as a method in the primary melting is an excellent melting method whereby alloying elements are melted due to an effect of a molten metal being stirred by the agency of an electromagnetic force following induction heating from a coil to thereby enable constituent adjustment to be easily effected so as to match a predetermined alloy composition. However, since in most cases, use is made of a melting vessel made of refractories based on oxides such as magnesia, alumina, and so forth, there is a risk in principle that oxygen is supplied from those refractories to a molten metal, so that there are limitations to oxygen removal.
Meanwhile, use of a halide-base refining flux such as calcium fluoride, calcium chloride, and so forth, in large amounts, is normally effective in removing impurities such as S, P, and so forth, that is, effecting desulfurization, and dephosphorization, against the molten metal. However, because halide-base refining agents will cause a crucible made of the oxide-base refractories to undergo intense melt-damage, use of this kind of refining flux is almost impossible in reality. Accordingly, with the current state of the art, high-purity raw materials must be used as melting raw materials in many cases when the vacuum induction melting method is adopted.
In contrast, the cold-crucible induction melting method often applied to melting of alloying materials that are quite active at a high temperature, such as Ti, Zr, and so forth, is characterized in that the water-cooled copper crucible is used in place of the crucible made of refractories. The method is advantageous in carrying out refining in a reducing atmosphere because Ca—CaF2 is used for a refining flux as a method for removal of S, and P, contained in stainless steel. The reason for that is because the refining flux such as Ca—CaF2, and so forth is harmless to the water-cooled copper crucible although the same will cause considerable damage on the crucible made of oxide-base refractories at a melting temperature.
In the following Patent Documents 1 to 3, and 6, 7, it is described that with this kind of method, P in a molten metal can be reduced to not higher than 5 ppm. Patent Documents 3, 6, and 7 describe a series of inventions based on the basic principle that use of flux is combined with cold-crucible type suspension melting to thereby cause oxide inclusions to migrate into the flux so as to be separated. Further, in Patent Document 2, it is described that when an extremely low P containing stainless steel is produced by the cold-crucible induction melting method, a Ca—CaF2 base refining flux is caused to be interjacent between hot metal and the water-cooled copper crucible, thereby causing P in the steel to migrate into the Ca—CaF2 flux.
Further, it is also known that if Ca out of Ca—CaF2, and so forth, for use in the refining flux for production of stainless steel, is mixed in the stainless steel, this will cause Ca ranging from several tens of ppm to several hundred ppm to remain in the stainless steel, thereby considerably deteriorating corrosion resistance thereof. In the following Patent Document 1, there is disclosed a method whereby an alloying material is first dephosphorized with a Ca—CaF2 base flux, to be treated again with a CaF2 flux, thereby reducing Ca content of an alloy to not more than 30 ppm, however, with this two-stage method, reaction efficiency is not so good as expected.
As a method for melting high-purity high-grade alloying materials by the cold-crucible induction melting method, the known basic methods for effecting dephosphorization, and decalcification by use of the Ca or CaF2 base flux as a refining agent have been introduced as above. Those methods certainly seem to be good methods in terms of dephosphorization, or decalcification, however, those methods are not considered satisfactory from the viewpoint of an object for comprehensively reduce, and remove a series of impurities, including C, O, N, S, P, and Ca, to the limits, as expected by the invention.
Further, besides the cold-crucible induction melting method described as above, the electron-beam melting method, and a vacuum melting method whereby the vacuum arc melting method using the water-cooled copper crucible is adopted have been in widespread use, however, any of those methods are intended for removal of specific elements, and non-metal inclusions, having not reached as yet a mature state for comprehensively reducing impurities such as C, O, N, S, P, Ca, and so forth.    Patent Document 1: JP-A No. 2003-55744    Patent Document 2: JP-A No. 2002-69589    Patent Document 3: JP-A No. 2000-248310    Patent Document 4: JP-A No. 2003-227687    Patent Document 5: JP-A No. 5 (1993)-322451    Patent Document 6: JP-A No. 11 (1999)-246919    Patent Document 7: JP-A No. 11 (1999)-246910